Three-way poppet valve for work exchanger

ABSTRACT

A three-way two-position poppet valve ( 10 ) comprising a housing ( 12 ) with a generally cylindrical valve chamber ( 14 ), a first ( 20 ) and a second ( 16 ) port with coaxial  1  valve seats ( 26, 28 ) at the ends of the chamber, and a third lateral working port ( 18 ). A poppet body ( 36 ) is disposed reciprocably in the valve chamber so that in a first position of the poppet body the first valve seat ( 26 ) is sealed and the second port communicates with the working port, and in a second position the second valve seat ( 28 ) is sealed and the first port communicates with the working port. The valve is characterized in that the housing has two coaxial cylindrical passages ( 24, 30 ) adjacent the respective valve seats, while the poppet body has two coaxial cylinder parts ( 44, 48 ) slidingly and sealingly fitting the cylindrical passages, so that the poppet body is always supported in at least one of the cylindrical passages and fluid communication between the first and the second port is always prevented.

FIELD OF THE INVENTION

This invention relates to three-way poppet valves, more particularly tovalves specially designed for use in work exchanger systems.

BACKGROUND OF THE INVENTION

A work exchanger is a device that obtains energy from one stream offluid and transfers that energy to another stream. It can be alsodescribed as a pump driven by fluid flow, most often of opposedpiston/diaphragm type. Work exchangers are vital for energy recovery inreverse osmosis processes such as desalination, since by itself the ROseparation is power-consuming process which becomes economicallyfeasible if only a substantial part of the energy resident in the rejector/and permeate streams is returned back into the process.

A work exchanger system typically comprises two (or more) pressurecylinder vessels with a brine port at one end, a feed water port at theother end, and a plunger freely sliding between the ports. A system ofvalves connects and disconnects these ports to high-pressure brine linecoming from RO modules, brine discharge line, low-pressure feed waterline, and high-pressure feed water line going to the RO modules. Eachpressure cylinder performs a two-stroke cycle whereby the energy of thehigh-pressure brine is transferred to the stream of feed water. Theresulting low-pressure brine is discharged.

At the first stroke, the brine port is connected to the brine dischargeline while the feed water port is connected to the low-pressure feedwater line. The vessel is filled with low-pressure feed water whichdisplaces the plunger towards the brine port and brine is dischargedthrough the non-pressurized discharge line.

At the second stroke, the brine port is connected to the high-pressurebrine line, while the feed port is connected to the high-pressure feedwater line. The vessel is filled with high-pressure brine whichdisplaces the plunger back towards the feed port so as to squeeze feedwater into the high-pressure feed water line.

The operation of the work exchanger requires special timing, reliablesynchronization and sealing of the valves in order to performefficiently the above two-stroke cycle.

A report on experimental work “A Flow Work Exchanger for DesalinationProcesses”, Kansas state Univ., Manhattan, August 1968, discloses usageof pilot-operated Hunt double plunger hydraulic valve with a workexchanger. This valve has a housing with two parallel cylinder bores andfour lateral ports opening into the bores. Two rigidly connectedparallel plungers are movable in the bores, providing communicationbetween the ports through specially formed channels and cavities inplunger bodies. The plungers are always in hydrostatic balance.

U.S. Pat. No. 5,306,428 to Tonner discloses a rotary valve used todirect brine to/from different work exchanger ports. The feed waterstream is regulated by two check valves at each feed water port. Therotary valve of Tonner is not hydraulically balanced, which causesexcessive wear on the sealing surfaces due to side loads exerted on thecentral rotating assembly. There are also internal and external leakageproblems between the high pressure inlet and outlet ports and the lowpressure drain ports. This, in turn, reduces the efficiency of theTonner valve and imposes size limits on any such device that can bemanufactured in practice.

U.S. Pat. No. 5,797,429 to Shumway suggests the usage of a five-way orfour-way linear spool valve in a work exchanger system. The Shumwayvalve comprises two pistons connected by a rod (spool) located inside acylinder. The cylinder has five ports: a high pressure brine inlet, afirst work exchanger vessel port, a second work exchanger vessel port,and two low-pressure brine discharge outlets which may be connected. Bymoving the spool back and forth within the cylinder, the work exchangerports are alternately opened and closed, and this directs flow in theproper sequence to the proper port. The feed water stream in the Shumwaywork-exchanger system is regulated by two check valves at the feed waterport of each exchanger vessel.

The linear spool valve of Shumway is hydraulically balanced axially. Asa result, the force required to move the linear spool is only that forceneeded to overcome the friction of the sealing surfaces associated withthe pistons, which permits the driving device of the valve to be of lowpower. However, the Shumway valve has also leakage problems. Theattempts to reduce leakage by tighter fitting of the pistons to thecylinder lead to excessive wear which seems to be an inherent problem inevery spool valve device because the sealing in spool valves is notprovided by positive displacement. This problem is even more aggravatedin work exchangers of large capacity and power that are employed inmodern desalination plants using RO technology.

Poppet valves have relatively simple design and provide very reliablesealing achieved by positive displacement. A typical three-way poppetvalve comprises a valve chamber with a central port and two coaxialvalve seats leading to two end ports, and a poppet body disposed in thevalve chamber. The poppet body is adapted for reciprocation between twopositions so that in a first position it seals the first valve seat andfluid communication is provided between one end port and the centralport, and in a second position the poppet body seals the second valveseat and fluid communication is provided between the other end port andthe central port. However, during the travel between the valve seats,the poppet body allows fluid communication between all three ports. Thepoppet valve also shuts-off and opens abruptly which may cause waterhammer, and is not hydraulically balanced.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a three-waytwo-position poppet valve comprising a housing with a first port, asecond port, a third working port and a generally cylindrical valvechamber with an axis. The valve chamber is defined between a firstcoaxial annular valve seat associated with the first port, and a secondcoaxial annular valve seat associated with the second port. The workingport is connected laterally to the valve chamber. The poppet valvefurther comprises a poppet body disposed in the valve chamber andadapted for reciprocation between two positions so that in a firstposition the poppet body seals the first valve seat and fluidcommunication is provided between the second port and the working port,and in a second position the poppet body seals the second valve seat andfluid communication is provided between the first port and the workingport.

The valve is characterized in that the housing has a first coaxialcylindrical passage adjacent the first valve seat and a second coaxialcylindrical passage adjacent the second valve seat, while the poppetbody has a first coaxial cylinder part slidingly and sealingly fittingthe first passage, and a second coaxial cylinder part slidingly andsealingly fitting the second passage, so that the poppet body is alwayssupported in at least one of the cylindrical passages and fluidcommunication between the first and the second ports is alwaysprevented. The cylindrical passages and the cylinder parts of the poppetbody preferably have the same diameter D.

In one embodiment of the three-way valve, the first valve seat is at adistal end of the first cylindrical passage, adjacent the first port,and a distal end of the first cylinder part of the poppet is equippedwith a first sealing rim matching the first valve seat. Preferably, thefirst cylindrical passage has a proximal part flaring towards the poppetbody so that the first cylinder part of the poppet body would smoothlychange the flow through the first port and the pressure in the valvechamber when entering or exiting the first cylinder passage.

The second valve seat is at a proximal end of the second cylindricalpassage, adjacent the valve chamber, and a proximal end of the secondcylinder part of the poppet is equipped with a second sealing rimmatching the second valve seat. Preferably, said poppet body furthercomprises a profiled part adjacent to the distal end of the secondcylinder part, so that the profiled part would smoothly change the flowthrough the second port and the pressure in the valve chamber whenentering or exiting the second cylinder passage.

The profiled part has a shape adapted to change flow section area of thesecond valve seat as a predetermined function of time for a givenvelocity of the poppet body axial movement. Preferably, the profiledpart comprises a shallow straight cylinder step adjacent to the distalend of the second cylinder part, the cylinder step having radial depthand axial length such that, after the second cylinder part leaves thesecond cylinder passage, pressures in the second port and in the workingport are equalized in a predetermined finite time for a given velocityof the poppet body axial movement.

In another embodiment of the three-way poppet valve, the second port isdisposed laterally to the axis and the housing further comprises anauxiliary coaxial cylinder chamber of diameter D communicating at aproximal end thereof with the second port and the second cylindricalpassage, and closed at a distal end thereof by a lid. An auxiliarypiston is mounted for sliding in the auxiliary chamber and is firmlyconnected to the poppet body by an axial rod. The auxiliary piston, theaxial rod and the poppet body form a poppet assembly which is axiallybalanced with respect to flow pressure in the second port.

The auxiliary piston sealingly fits the auxiliary cylinder chamber,thereby defining a sealed volume between the lid and the auxiliarypiston. Preferably, the sealed volume is provided with fluidcommunication to the first port, so that pressures acting on the poppetassembly from the sealed volume and from the first port are equalized.The fluid communication may be provided by an external pipe connectingthe sealed volume to the first port or by a channel made in the rod.

According to another aspect of the present invention, there is provideda work exchanger module adapted to utilize the energy of high-pressureworking fluid for pumping feed fluid, comprising an exchanger cylinderwith a first working fluid end and a second feed fluid end, and a pistonfreely sliding therebetween. A first three-way poppet valve as above isconnected by its working port to the first end of the exchangercylinder, by its second port to a source of high-pressure working fluidand by its firs port to a non-pressurized discharge outlet. A secondthree-way poppet valve is connected by its working port to the secondend, by its first port to a source of low-pressure feed fluid and by itssecond port to a high-pressure feed fluid consumer.

According to yet another aspect of the present invention, the two poppetvalves of the above work exchanger module are equipped with a first anda second hydraulic cylinder, each having a “push” and a “pull” port,such that connecting the “push” port to a pressure source would drivethe associated poppet valve to the first position thereof andvice-versa, wherein the “push” port of the first hydraulic cylinder isconnected directly to the “pull” port of the second hydraulic cylinder.Thereby, the reciprocating motions of the two poppet valves are fullysynchronized.

Preferably, for such synchronized motion, the flaring part of the firstcylindrical passage in the first three-way poppet valve is axiallylonger than the corresponding flaring part in the second three-waypoppet valve, so that when the two poppet bodies move towards openingthe first port in both three-way valves, the first port in the firstvalve is connected to the exchanger cylinder before the first port ofthe second valve.

The poppet valve of the present invention provides combined advantagesof poppet and spool valves: reliable sealing of the ports, avoidingmixing of flows and leakages, controlled shut-off and opening of theflow passages and preventing water-hammer, axially balanced poppetassembly allowing low-power drives, reduced use of expensive materials,robust construction and low production costs.

The three-way poppet valve of the present invention is advantageouslyused in energy recovery plants of RO installations of large capacity. Itallows constructing of large work exchanger units where a number ofexchanger cylinders are connected in parallel to one poppet valve.Opposite-phase operation of two work exchangers, which is necessary insuch energy recovery plant, can be effectively synchronized by designingthe profile of the poppets such that while one valve opens itshigh-pressure brine port and the other valve closes its high-pressurebrine port, the total sectional area of the high-pressure brine flowremains constant.

The three-way poppet valve of the present invention, with the hydraulicdrive, can be advantageously used also at the feed water end of the workexchangers, instead of two check valves. Thus, with no springs or otherelastic elements used in valve motion, arbitrary closing/opening or“hesitation” of the valves is avoided, as well as noise and “waterhammer”.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic cross-sectional elevation of a three-way balancedpoppet valve of the present invention, in a first extreme position ofthe poppet.

FIG. 2 is a schematic cross-sectional elevation of the three-waybalanced poppet valve of FIG. 1 in a second extreme position of thepoppet.

FIG. 3 is a schematic cross-sectional elevation of a work-exchangermodule equipped with two three-way poppet valves of the presentinvention.

FIG. 4 is a scheme of a RO desalination plant with energy recovery plantcomprising the work exchanger of FIG. 3.

FIGS. 5A and 5B show alternative embodiments of the three-way poppetvalve of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1 and 2, there is shown a three-way,two-position poppet valve 10 comprising a housing 12 with a generallycylindrical valve chamber 14, a first (outlet) port 20, a second (inlet)port 16, a third (working) port 18, and a poppet assembly 22. The valvechamber 14 has a coaxial cylinder passage 24 of diameter D towards theinlet port 16. The cylindrical passage 24 comprises a first valve seat26 of diameter D disposed at the distal end thereof, in communicationwith the axial outlet port 20, and a flaring annular part 27 at theproximal end. A second annular valve seat 28 is at the opposite side ofthe valve chamber 14, communicating with the inlet port 16. The workingport 18, which is disposed laterally between the valve seats 26 and 28,is directly communicating with the valve chamber 14. A coaxial cylinderpassage 30 of diameter D is provided between the valve seat 28 and theinlet port 16. The housing 12 further has an auxiliary coaxial cylinderchamber 32 of diameter D adjacent to and communicating with the inletport 16, the auxiliary chamber being closed by a lid 34. It should benoted that the first port 20 and the second port 16 are namedhere“outlet” and “inlet” just for convenience while either of them canbe inlet or outlet.

The poppet assembly 22 comprises a poppet body 36 with an axial stem 38and an auxiliary piston 40. The poppet body 36 comprises a metal dish 42fixed to the stem 38 and having a sealing rim with an annular seal 44, afirst cylinder body 46 with a metal sealing ring 52, and a secondcylinder body 48 with a profiled, generally tapering, extremity 50. Boththe flaring part 27 and the extremity 50 profile are designed forobtaining smooth flow at opening and closing the first and the secondport, respectively. Their particular form depends on the application ofthe valve as will be explained below. The cylinder bodies 46 and 48 fitslidingly and sealingly into the cylindrical passages 24 and 30,respectively. The auxiliary piston 40 fits sealingly into the auxiliarychamber 32, thereby defining a balancing pressure chamber 54 between thepiston 40 and the lid 34. The pressure chamber 54 is in fluidcommunication with the outlet port 20 via a pipe 56 (shown in FIG. 1)or, alternatively, via a channel 58 obtained through the stem 38 (shownin FIG. 2). The axial stem 38 extends, with a sealing sliding fit,through an opening in the lid 34 and is connected to an externalhydraulic cylinder 60.

In operation, the hydraulic cylinder 60 reciprocates the poppet assembly22 between two extreme positions: in a first position (shown in FIG. 2),the first valve seat 26 is sealed by the sealing ring 52 and fluidcommunication is provided between the second (inlet) port 16 and theworking port 18 under pressure P₂; and in a second position (shown inFIG. 1), the second valve seat 28 is sealed by the sealing rim 44 andfluid communication is provided between the working port 18 and thefirst (outlet) port 20 under pressure P₁.

During the reciprocating motion, the poppet assembly 22 is supported bythe cylinder bodies 46 and 48 sliding in the cylinder passages 24 and30. The axial lengths of the cylinder bodies and the cylinder passagesare selected so that cylinder passages 24 and 30 are never openedsimultaneously and thus flows via the first port 20 and the second port16 are not mixed. These axial lengths may be further varied if necessaryfor adjusting the valve operation cycle (timing) to the working cycle ofa machine where the valve is used.

The profiled extremity 50 of the poppet body has a shape adapted tochange flow section area of the second valve seat 28 as a predeterminedfunction of time for a given velocity of the poppet body axial movement.For example, if two valves 10 are used with two work exchange cylindersoperating in opposite phase (see FIG. 4, valves 80 and 80′), theextremities of their respective poppet bodies may be shaped so that whenone valve opens its second port and the other valve closes its secondport, the total sectional area of the flow through these second portsremains constant.

The profile of the extremity 50 has a shallow straight cylinder step 51adjacent to the second cylinder body 48, with depth d and axial lengthl. These dimensions are selected such that, when the second cylinderbody 48 leaves the second cylinder passage 30 opening it, pressures inthe second port 16 and in the working port 18 are equalized to P₂ in apredetermined finite time for a given velocity of the poppet body axialmovement. This timing is necessary in order to avoid occurring of “waterhammer”. It will be appreciated that particular dimensions of the step51 depend also on the pressure differential between ports 16 and 18before opening of the valve seat 28, on the volume of a fluid containerfilled through the working port 18, and on the elastic properties ofthis container, the associated piping, and of the fluid. For example(see FIG. 3), a work exchanger vessel 72 of volume V connected to apoppet valve 80 may operate under high pressure P₂=70-80 ata and expandits volume by ΔV when exposed to such pressure. The vessel 72, the watertherein, and the connecting piping therearound constitute an elasticoscillating system characterized by basic natural frequency f or periodT. Thus, it is desirable to feed the volume of water ΔV into the vessel,under the pressure P₂, after the valve seat 28 is opened, for a timeequal or longer than half the period T. This time can be attained byselecting the depth d and the axial length l of the step 51.

With reference to FIG. 2, in the first extreme position of the poppet,axial forces applied to the poppet assembly 22 from the pressure P₂within the assembly are mutually balanced, since the chamber 32 and thepassage 24 have the same diameter D. These forces are balanced duringthe whole travel of the poppet assembly towards the second extremeposition and in the second position (FIG. 1).

The axial pressures acting from the balancing pressure chamber 54 ontothe piston 40 and from the outlet port 20 onto the cylinder body 46 areboth equal to P₁ due to the fluid communication 56 (or 58) thattransmits the pressure of the outlet port 20 to the chamber 54. Howeverin the first position (FIG. 2), the area of the poppet body exposed tothe axial pressure P₁ in the valve seat 26 is slightly less than πD²/4because the annular seal contact surface has finite width extendinginside of the diameter D. (This width can be minimized by the channel 53in the sealing ring 52). Conversely, in the second position (FIG. 1),the area of the poppet body exposed to pressure P₁ in the valve seat 28is slightly more than πD²/4 because the annular contact surface of theseal 44 has finite width extending outside of the diameter D. Betweenthe extreme positions, the area of the poppet body exposed to the axialpressure P₁ from the outlet port 20 is equal to πD²/4. The area of thepiston 40 exposed to pressure P₁ is also less than πD²/4 by the area ofthe rod 38 cross-section.

The above balancing scheme is especially advantageous for use with thepressure in the second port much higher than the pressure in the firstport (P₂>>P₁) since all unbalance due to differences of areas isassociated with the lower pressure. Thus, the construction of thethree-way poppet valve of the present invention allows the poppetassembly to be reciprocated without overcoming pressure differentials ofthe inlet and outlet flows, while the constant diameter D facilitatesworking and finishing of the bores 24, 30 and 32 in the housing 12.

With reference to FIGS. 3 and 4, there are shown two identical workexchanger modules 70 and 70′ used in a power recovery system 62connected to a reverse osmosis desalination plant 64 (RO plant). Thepower recovery system 62 utilizes the energy of high-pressure brine forpumping feed water to the RO plant.

The module 70 comprises an exchanger cylinder 72 with a brine port 74 atone end, a feed water port 76 at the second end, and a plunger 78 freelysliding between the ports. The module 70 is equipped with two three-waypoppet valves, as described in relation to FIGS. 1 and 2. The firstthree-way poppet valve 80 is connected by its working port 82 to thebrine port 74, by its second (inlet) port 84 to a high-pressure brineline 86 of the RO plant, and by its first (outlet) port 88 to anon-pressurized brine discharge line 89. The second three-way poppetvalve 90 is connected by its working port 92 to the feed water port 76,by its second (outlet) port 94 to a high-pressure feed line 96 of the ROplant, and by its first (inlet) port 98 to a low-pressure feed line 99.In the position of the poppet valves 80 and 90 shown in FIG. 3, the workexchanger module 70 receives low-pressure feed water and dischargesbrine. The work exchanger module 70′ has its valves 80′ and 90′ inreverse position where high-pressure brine is fed into the cylinder andpumps high-pressure feed water into the RO plant.

With reference only to FIG. 3, the first poppet valve 80 has a hydrauliccylinder 100 with ports “pull” 102 and “push” 104, the second poppetvalve 90 has a hydraulic cylinder 110 with ports “pull” 112 and “push”114, and the hydraulic cylinders are powered by a hydraulic station 120with inlet port 122 and outlet port 124. The “pull” port 102 of thecylinder 100 is connected directly to the “push” port 114 of the otherhydraulic cylinder 110, while the “push” port 104 of the cylinder 100 isconnected to the hydraulic station outlet 124 and the “pull” port of thecylinder 110 is connected to the hydraulic station inlet 122. The line102-114, which is not connected to the hydraulic station, has means forcompensation of leakage 126.

It would be appreciated that in this way the reciprocation of the twopoppet valves is synchronized: When high pressure is fed to thehydraulic station outlet 122, the piston of the cylinder 110 and thepoppet valve 90 are pulled to the position of FIG. 3 whilesimultaneously the cylinder 100 and the poppet valve 80 are pulled tothe same position due to the connection 114-102. The reverse motion isalso synchronized. This method of synchronization is suitable for usewith sensors (not shown) for measuring position and speed of the plunger78.

The first three-way poppet valve 80 may have a flaring annular part 87of the cylindrical passage 24 which is axially longer than thecorresponding flaring part 97 in the second poppet valve 90, so thatduring synchronized motion of the two poppet bodies towards opening thefirst ports 88, 98 in both three-way valves, the first port 88 in thefirst valve 80 is connected to the exchanger cylinder 72 before thefirst port 98 of the second valve 90. This is done in order to reliefthe high pressure in the exchanger cylinder 72 from the previous(second) stroke into the brine discharge line 89 (see also FIG. 4) andnot into the low-pressure feed line 99.

Although a description of specific embodiments has been presented, it iscontemplated that various changes could be made without deviating fromthe scope of the present invention. For example, the poppet valve ofFIG. 1 could be modified as shown in FIGS. 5A and 5B. In FIG. 5A, athree-way poppet valve 130 has a cylinder passage 24′ at the axialoutlet 20 designed symmetrically to the cylinder passage 30, with avalve seat 26′ similar to the valve seat 28 (see FIG. 1). The poppetbody 36′ in this case has symmetrical form with a cylindrical body 46′complemented by a profiled extremity 50′, and a sealing rim 52′ similarto the rim 44. Alternatively, as shown in FIG. 5B, a three-way poppetvalve 140 may have a cylinder passage 30′ designed symmetrically to thecylinder passage 24, with valve seat 28′ at the distal end of thepassage 30′, and a flaring annular part 27′ at the proximal end. Thepoppet body 36″ in this case is a symmetrical cylinder without profiledextremities. The second sealing rim 44′ is similar to the sealing rim52. These valve variations, however, can not be balanced completely withrespect to either pressure P₂ or P₁.

1. A three-way two-position poppet valve comprising a housing with a first port, a second port, a third working port and a generally cylindrical valve chamber with an axis, said valve chamber being defined between a first coaxial annular valve seat associated with said first port, and a second coaxial annular valve seat associated with said second port, said working port being connected laterally to said valve chamber, said poppet valve further comprising a poppet body disposed in said valve chamber and adapted for reciprocation between two positions so that in a first position the poppet body seals said first valve seat and fluid communication is provided between said second port and said working port, and in a second position the poppet body seals said second valve seat and fluid communication is provided between said first port and said working port; wherein said housing has a first coaxial cylindrical passage adjacent said first valve seat and a second coaxial cylindrical passage adjacent said second valve seat, said poppet body has a first coaxial cylinder part slidingly and sealingly fitting said first passage, and a second coaxial cylinder part slidingly and sealingly fitting said second passage, so that said poppet body is always supported in at least one of the cylindrical passages and fluid communication between said first and said second ports is always prevented.
 2. The three-way poppet valve of claim 1, wherein the two coaxial cylindrical passages and the two coaxial cylinder parts of the poppet body have the same diameter D.
 3. The three-way poppet valve of claim 2, wherein said second valve seat is at a proximal end of said second cylindrical passage, adjacent said valve chamber, and a proximal end of said second cylinder part of the poppet is equipped with a second sealing rim matching said second valve seat.
 4. The three-way poppet valve of claim 3, wherein said poppet body further comprises a profiled part adjacent to the distal end of said second cylinder part, so that said profiled part would smoothly change the flow through the second port and the pressure in the valve chamber when entering or exiting said second cylinder passage.
 5. The three-way poppet valve of claim 4, wherein said profiled part comprises a shallow straight cylinder step adjacent to the distal end of said second cylinder part, said cylinder step having radial depth and axial length such that, after said second cylinder part leaves said second cylinder passage, pressures in said second port and in said working port are equalized in a predetermined finite time for a given velocity of the poppet body axial movement.
 6. The three-way poppet valve of claim 4, wherein said profiled part has a shape adapted to change flow section area of the second valve seat as a predetermined function of time for a given velocity of the poppet body axial movement.
 7. The three-way poppet valve of claim 3, wherein said first valve seat is at a distal end of said first cylindrical passage, adjacent said first port, and a distal end of said first cylinder part of the poppet is equipped with a first sealing rim matching said first valve seat.
 8. The three-way poppet valve of claim 7, wherein said first cylindrical passage has a proximal part flaring towards said poppet body so that said first cylinder part of the poppet body would smoothly change the flow through the first port and the pressure in the valve chamber when entering or exiting said first cylinder passage.
 9. The three-way poppet valve of claim 8, wherein said second port is disposed laterally to said axis and said housing further comprises an auxiliary coaxial cylinder chamber of diameter D communicating at a proximal end thereof with said second port and said second cylindrical passage, and closed at a distal end thereof by a lid, an auxiliary piston being mounted for sliding in said auxiliary chamber and being firmly connected to said poppet body by an axial rod, so that the auxiliary piston, the axial rod and the poppet body form a poppet assembly which is axially balanced with respect to flow pressure in the second port.
 10. The three-way poppet valve of claim 9, wherein said auxiliary piston sealingly fits said auxiliary cylinder chamber, thereby defining a sealed volume between said lid and said auxiliary piston, said sealed volume being provided with fluid communication to said first port, so that pressures acting on the poppet assembly from said sealed volume and from said first port are equalized.
 11. The three-way poppet valve of claim 10, wherein said axial rod extends, with a sealing sliding fit, through an opening in said lid and is connectable to an external drive means for moving the poppet assembly between the two positions thereof.
 12. The three-way poppet valve of claim 11, wherein said drive means is a hydraulic cylinder connected to said rod.
 13. The three-way poppet valve of claim 10, wherein said fluid communication is provided by an external pipe connecting said sealed volume to said first port.
 14. The three-way poppet valve of claim 10, wherein said fluid communication is provided by a channel obtained through said axial rod.
 15. A work exchanger module adapted to utilize the energy of high-pressure working fluid for pumping feed fluid to a feed fluid consumer, comprising an exchanger cylinder with a first working fluid end and a second feed fluid end, a piston freely sliding therebetween, and a first three-way poppet valve according to claim 10, said poppet valve being connected by its working port to said first end, by its second port to a source of high-pressure working fluid and by its first port to a non-pressurized discharge outlet.
 16. A work exchanger module adapted to utilize the energy of high-pressure working fluid for Pumping feed fluid to a feed fluid consumer, comprising an exchanger cylinder with a first working fluid end and a second feed fluid end, a piston freely sliding therebetween, and a first three-way poppet valve comprising a housing with a first port, a second port, a third working port and a generally cylindrical valve chamber with an axis, said valve chamber being defined between a first coaxial annular valve seat associated with said first port, and a second coaxial annular valve seat associated with said second port, said working port being connected laterally to said valve chamber, said poppet valve further comprising a poppet body disposed in said valve chamber and adapted for reciprocation between two positions so that in a first position the poppet body seals said first valve seat and fluid communication is provided between said second port and said working port, and in a second position the poppet body seals said second valve seat and fluid communication is provided between said first port and said working port; wherein said housing has a first coaxial cylindrical passage adjacent said first valve seat and a second coaxial cylindrical passage adjacent said second valve seat, said poppet body has a first coaxial cylinder part slidingly and sealingly fitting said first passage, and a second coaxial cylinder part slidingly and sealingly fitting said second passage, so that said poppet body is always supported in at least one of the cylindrical passages and fluid communication between said first and said second ports is always prevented, said poppet valve being connected by its working port to said first end, by its second port to a source of high-pressure working fluid and by its first port to a non-pressurized discharge outlet, further comprising a second three-way poppet valve according to claim 10, said second poppet valve being connected by its working port to said second end, by its first port to a source of low-pressure feed fluid and by its second port to a high-pressure feed fluid consumer.
 17. The work exchanger module of claim 16, wherein the axial rods of the two poppet valves are extending outside the housings, the module further comprising a first hydraulic cylinder connected to the rod of the first poppet valve for driving thereof and a second hydraulic cylinder connected to the rod of the second poppet valve for driving thereof.
 18. The work exchanger module of claim 17, where said hydraulic cylinders have each a “push” and a “pull” port, such that connecting the “push” port to a pressure source would drive the associated poppet valve to the first position thereof and connecting the “pull” port to a pressure source would drive the associated poppet valve to the second position thereof, wherein the “push” port of the first hydraulic cylinder is connected directly to the “pull” port of the second hydraulic cylinder.
 19. The work exchanger module of claim 18, wherein the flaring part of the first cylindrical passage in said first three-way poppet valve is axially longer than the corresponding flaring part in said second three-way poppet valve, so that during synchronized motion of the two poppet bodies towards opening the first port in both three-way valves, the first port in the first valve is connected to the exchanger cylinder before the first port of the second valve.
 20. A three-way two-position poppet valve comprising a housing with a first port, a second port, a third working port and a generally cylindrical valve chamber with an axis, said valve chamber being defined between a first coaxial annular valve seat associated with said first port, and a second coaxial annular valve seat associated with said second port, said working port being connected laterally to said valve chamber, said valve housing having a first coaxial cylindrical passage adjacent said first valve seat and being of smaller radial dimension therefrom, and a second coaxial cylindrical passage adjacent said second valve seat and being of a smaller radial dimension therefrom; said poppet valve further comprising a poppet body disposed in said valve chamber and adapted for reciprocation between two positions so that in a first position the poppet body seals said first valve seat by axial pressure thereagainst and fluid communication is provided between said second port and said working port, and in a second position the poppet body seals said second valve seat by axial pressure thereagainst and fluid communication is provided between said first port and said working port; wherein said poppet body has a first coaxial cylinder part slidingly and sealingly fitting said first passage, and a second coaxial cylinder part slidingly and sealingly fitting said second passage, so that said poppet body is always supported in at least one of the cylindrical passages and fluid communication between said first and said second ports is always prevented.
 21. A three-way poppet valve according to claim 20, said first valve seat is at a distal end of said first cylindrical passage, adjacent said first port, and a distal end of said first cylinder part of the poppet is equipped with a first sealing rim matching said first valve seat so as to provide said axial pressure, and said second valve seat is at a proximal end of said second cylindrical passage, adjacent said valve chamber, and a proximal end of said second cylinder part of the poppet is equipped with a second sealing rim matching said second valve seat so as to provide said axial pressure. 